The invention relates to an optical device comprising a diode laser for generating a radiation beam, and an optical system for concentrating and guiding the radiation beam.
The invention also relates to an apparatus for scanning an information plane, which apparatus comprises such a device.
An information planar structure or surface is herein understood to mean a plane comprising information which can be read optically, or such a plane in which information can be written optically. Such a plane is, for example the information plane of an optical record carrier, but also the surface of a document which is written with a laser printer or is read with a document scanner, as in a facsimile apparatus.
A device of the type described in the opening paragraph is known, for example from European Patent Application EP 0.084.871 which relates to an apparatus for reading an optical record carder. The diode lasers used in this apparatus are very sensitive to feedback. Due to reflection of the laser radiation from elements of the optical system, a portion of the radiation emitted by the diode laser may return to the active layer of the diode laser. A small quantity of radiation fed back, even if only of the order of 0.1% of the emitted radiation, may cause a variation of the laser radiation. Dependent on the quantity of radiation fed back, this may lead to an increase of the line width of the laser, wavelength variation in the output spectrum, or noise. These phenomena are generally undesirable. Moreover, the effects are generally not constant but dependent on the phase of the light returning to the laser; in other words, they are dependent on optical path length variations of the order of the laser wavelength, for example, due to displacements or vibrations of optical elements.
It is therefore attempted to avoid feedback in the laser as much as possible. However, practice has proved that, inter alia due to tolerances of the elements in the optical system, the feedback cannot be prevented to a sufficient extent, unless very stringent requirements are imposed on the optical elements so that the device will be too expensive for various uses.
It is proposed in European Patent Application 0.084.871 to increase the quantity of radiation fed back to the order of 1 to 10% of the radiation emitted by the diode laser in a device in which a diode laser emits radiation continuously. This measure is based on the recognition that the noise of the laser radiation caused by the feedback does not increase continuously with the quantity of radiation which has been fed back, but that there is a given level for this radiation at which the noise level is maximum and the quantity of noise decreases with a further increase of the feedback. However, in the device with the continuously emitting diode laser the intensity of the laser beam will decrease due to an increase of the feedback, which is particularly detrimental for, for example, a device for writing optical record carriers. Moreover, as a result of the feedback the output spectrum will shift in wavelength, which is very undesirable for uses in which the operation of a system is adjusted to a given wavelength band.